1. Field of the Invention
The present invention relates to a shock sensing system that disables the write gate of a hard disk drive when the disk drive is subjected to an excessive shock load.
2. Description of Related Art
Hard disk drives contain one or more magnetic disk which are typically organized into a plurality of annular tracks. Data is stored within the disk drive by locating a magnetic head above a particular track and then magnetizing the disk. The magnetic head is mounted to an actuator arm which can move the head to any one of the track locations. The tracks may have segments which contain coded information that uniquely identify each track. Proper track location can be obtained by reading the coded information within the track segments and moving the magnetic head accordingly.
To reduce system errors, it is desirable to locate the magnetic head within the boundaries of each track during the read and write operations of the disk drive. If the magnetic head is moved toward an adjacent track by an external disturbance, the data in the adjacent track can be corrupted if a write operation is in progress. For example, if the magnetic head moves while the system is writing, the new data may write over the old data on the adjacent track, resulting in an unrecoverable loss of the old data.
Present hard disk drives typically prevent head movement by employing either a continuous servo system or a sample servo system. In a sample servo system, each track contains a segment(s) which has embedded signals that are interpreted by the control circuitry to determine if the head is not aligned with the track. If the head is off-center, the control circuit moves the actuator arm accordingly. As the disk spins, the servo segments periodically rotate below the heads, so that the head location can be periodically computed and adjusted.
A continuous servo system is typically employed in a multi-disk drive unit. A continuous system has a single head and an entire disk surface dedicated to sensing the location of the heads. The servo head is rigidly attached to the other magnetic heads, so that the head locations can be constantly monitored.
Hard disk drives used in portable computers are particularly susceptible to external shock and vibrational loads. An excessive shock or vibrational load may cause the magnetic head to move to an adjacent track. If this head movement occurs while the drive is writing data, the old data on the adjacent track will be lost. It is therefore desirable to have a disk drive unit which prevents data from being lost when the disk drive is subjected to an external load. Because of the periodic nature of sample servo systems, such systems are too slow to prevent at least some data from being lost, particularly if a high frequency shock were to occur while the head was between the servo segments of the track. Although continuous servo systems provide a faster response, such systems require a dedicated disk surface which reduces the storage space of the overall system.
Application Ser. No. 07/976,441 filed on Nov. 13, 1992 and assigned to the same assignee as the present invention, discloses a system which disables the write gate of the transducer when the disk drive is subjected to a shock load that exceeds a predetermined value. The system contains a mechanical transducer which senses any acceleration of the disk drive. The transducer provides an output signal which is coupled to a circuit that disables the write gate if the signal exceeds a threshold value. Although effective in sensing shock, mechanical transducers occupy valuable space within the disk drive. Additionally, the transducer is mounted to the base plate of the drive and therefore does not provide an accurate indication of the relative movement between the actuator arm and the disk. It would therefore be desirable to provide an accurate shock sensing system that would require a minimal amount of space.